This invention relates, generally, to non-aqueous electrochemical cells, and more particularly to inorganic cells employing an alkaline metal, such as lithium, as the anode with a cathode or cathode collector separated from the anode by a separator membrane. Cells of this nature have been provided either in the form of flat parallel plates of anode, separator and cathode material, or as spirally wound elements nested within a suitable container.
Commercial electrochemical cells referred to as lithium/SO.sub.2 cells, typically contain lithium anodes and electrolytes comprised of a salt such as LiBr dissolved in a solvent mixture of liquified SO.sub.2 and an organic co-solvent, such as acetonitrile. Cathodes of such cells are usually comprised of carbon black, such as Shawinigan (acetylene black), formed on an expanded metal substrate.
Discharge of such cells results in the formation of anode metal dithionite at the carbon cathode surface. In such cells, the SO.sub.2 fluid cathode depolarizer acts directly with the anode metal cation to form such dithionite product. Since the cathode reaction is a direct one between the anode and fluid cathode depolarizer, the carbon cathode collector only provides a catalytic surface for such reaction.
The high energy density of such cells permits high currents to be drawn from them, particularly when the electrodes are spirally wound together. Such a cell design, while permitting high current drains, is susceptible to being rendered unsafe when subjected to certain conditions. For example, it is very difficult to manufacture cells having identical capacities. Therefore, when cells are discharged in series, it is quite possible that one cell will exhaust its capacity before the others. Also, during overcharge, the cell is caused to be in a state of voltage reversal, causing electrochemical reactions to occur which generate heat and lead to bulging or venting.
It is recognized that non-aqueous electrochemical cells having lithium or similar alkaline metal anodes commonly have a lower cycle life than comparable aqueous systems which employ cadmium or lead negative electrodes. A major cause of the death of such lithium cells is the formation of dendrites which grow from the lithium electrode and make electronic contact with complementary positive electrodes.
There have been various teachings of the problems resulting from dendrite growth between the anode and cathode. One such teaching can be found in U.S. Pat. No. 4,622,277 dated Nov. 11, 1986. In this reference, it is taught to use an exposed inert conductive metal coupled mechanically and electrically to the cathode and coupled mechanically and electrically to the anode. It is taught that when these electrodes are spirally wound, the two pieces of inert metal are oriented such that they face each other and are held in physical isolation by the separator which is interposed therebetween. During voltage reversal abuse, dendrites grow from the first segment of inert metal to the dendrite target, thereby creating a low resistance pathway between the two pieces of inert metal. It is taught that this prevents the potentially detrimental intermixing of anode material into the cathode to provide a shunt for the current to pass through the reverse cell without generating excessive heat.
Although approaches such as those shown in U.S. Pat. No. 4,622,277 work effectively to avoid venting in primary non-aqueous electrochemical cells, such solutions are not appropriate when dealing with secondary or rechargeable systems. There are various reasons why primary cell technology cannot be extended directly to prevent venting in secondary cells. For example, there is normally a dendrite-resistant separator present between electrodes in a secondary cell to prevent lithium shorts during charge. Further, solutions are normally selected for the capability of depositing non-dendritic lithium deposits in a secondary system. Common electrolytic solutions for use herein contain, for example, the salt of lithium aluminum tetrachloride and sulfur dioxide.
Cells of the type contemplated for use herein generally are provided with a cathode or cathode collector of a carbonaceous material and further containing a positive active material such as cupric chloride. This positive active material encourages dendrite growth and in the event of the elimination of a microporous separator which was resistant to the passage of alkaline metal dendrites, the cell would tend to short during normal discharge. As such, it has been found that the cell must be provided with over-discharge protection in an area remote from the cell stack.
It is thus an object of the present invention to provide a non-aqueous electrochemical cell having been prevented from venting by providing a mechanism to create a dendritic short external to the cell stack during excessive over-discharge.